It is of very great importance in the manufacture of semiconductor devices, and in other chemical process operations, to control and/or measure the rate of inflow of a reagent into a chemical reactor. In the semiconductor industry, the common practice is to place silicon wafers supported in specially designed holders in a vacuum furnace, heat the wafers and, in successive operations, inflow into the furnace a reagent or a mixture of reagents at very precisely controlled mass flow rates where the reagent(s) react with the wafers or portions of the wafer and form alloys, coatings, or interstitial silicon compounds on the wafer surface. Such operations are described in considerable detail in Wolf, S. and Tauber, R. N. SILICON PROCESSING FOR THE VLSI ERA, Lattice Press, Sunset Beach, Calif. (1987). Other processes to which this invention is applicable are described by Considine, D. M., (Ed.), CHEMICAL AND PROCESS TECHNOLOGY ENCYCLOPEDIA, McGraw-Hill, New York, N.Y. and in many other encyclopedic works and in technical journals such as Chemical Engineering and the numerous trade journals which relate to the semiconductor trade and to other chemical process industries.
A number of methods and apparatuses for controlling reagents have been described. One of such instruments which has gained wide acceptance is the M-DOT.RTM. mass flow controller manufactured by the J. C. Schumacher Company, Oceanside, Calif. Other flow controllers include a micro-syringe drive controller sold by Houston Atlas, Inc. Houston, Tex. which is a mechanical system in which the liquid is pumped by a micro-syringe, and a thermal pulse device in which a heat pulse is imparted to the liquid, one form of which is sold by Molytek, Inc. Pittsburgh, Pa. See also, Albert, H. J. and Wood, R. J., Rev. Sci. Instrum. 56(10), Oct. 1985, and Miller, T. E. Jr. and Small, H., Analytical Chemistry 1982, 54.907, which describe thermal pulse and time of flight devices.